High linearity inductorless LNA

ABSTRACT

An inductor-less low noise amplifier (LNA) with high linearity is disclosed. The low noise amplifier includes: an input signal stage receiving an input signal; a first amplifier configured to receive the input signal, generate a first amplification signal by amplifying the received input signal, and output the generated first amplification signal, as a first output signal, to a first output terminal; a second amplifier configured to receive the input signal, generate a second amplification signal by amplifying the received input signal, and output the generated second amplification signal, as a second output signal, to a second output terminal; an output signal stage outputting a superimposition signal obtained by superimposing the first output signal and the second output signal; a first resistor feeding back the superimposition signal to the input signal stage; and a switch connecting/disconnecting between the input signal stage and the output signal stage.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an inductor-less LNA (Low Noise Amplifier) with a blocking function which is used in a communication system.

Background of the Invention

This section is just to provide background information related to embodiments of the present invention but is not intended to explain conventional techniques related thereto.

As is known in the related art, ISDB-T (Integrated Service Digital Broadcasting Terrestrial) uses a UHF (Ultra High Frequency) frequency band (470 MHz to 707 MHz), ISDB-Tmm (Integrated Service Digital Broadcasting Terrestrial Mobile Multi-Media) uses a VHF (Very High Frequency) frequency band (170 MHz to 280 MHz), and ISDB-Tsb (Integrated Service Digital Broadcasting Terrestrial Sound Broadcasting) uses an LVHF (Low Very High Frequency) frequency band (90 MHz to 110 MHz). Among these, the ISDB-Tmm scheme has been devised for mobile broadcasting based on the ISDB-T scheme which is being currently used for terrestrial digital TV broadcasting. That is, the ISDB-Tmm scheme can offer a fused service for two media, i.e., communication and broadcasting having different properties.

In mobile tuner environments supporting the above-mentioned ISDB-T, ISDB-Tmm and ISDB-Tsb, a TV receiving chip and an antenna are relatively distant from each other. This may produce a line loss, which may result in increase in NF (Noise Figure) and hence deterioration of reception sensitivity.

In order to minimize such deterioration of reception sensitivity, there has been conventionally proposed a method for additionally using a small capacity low noise amplifier disposed immediately before the antenna to increase the reception sensitivity of the TV receiving chip against the line loss. However, recent use of an LTE (Long Term Evolution) band ranging from 700 MHz to 2.4 GHz for mobile phones deteriorates the performance of the ISDB-T of the UHF frequency band (470 MHz to 707 MHz). The reason for the performance deterioration will be described below with reference to FIG. 1.

FIG. 1 shows a channel environment of a conventional LNA.

Referring to FIG. 1, in a channel environment for a mobile TV tuner, a desired channel has less power, whereas ambient channels have more power. Therefore, in order to avoid such power unbalance, there is a need to increase the linearity of the LNA such that harmonic tones of the ambient channels have no effect on the desired channel.

If the LNA supports ISDB-Tmm, in general, two amplifiers are used to support a frequency band. An LNA may be used to support a UHF frequency band and a VHF frequency band in a wide range. However, in mobile environments, if a large input signal with jammer power occurs in a frequency band adjacent to the desired channel, an interference may occurs between signals of the desired channel. Therefore, there is need to reduce an interference between adjacent frequency bands. In addition, there is a need to reduce an interference between frequencies of FM signals used between a UHF frequency band and a VHF frequency band and an interference between UHF adjacent frequency bands such as CDMA (Code Division Multiple Access), LTE (Long Term Evolution) and the like.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an inductor-less low noise amplifier (LNA) with a first amplifier and a second amplifier, which is capable of making IC (Integrated Circuit) more compact and improving a noise figure (NF) by increasing trans-conductance of the LNA by reusing a current of the first amplifier in the second amplifier.

It is another object of the present invention to provide a low noise amplifier which is capable of achieving high linearity by cancelling the third-order trans-conductance of the first amplifier and the third-order trans-conductance of the second amplifier from each other in order to minimize a third-order intermodulation component.

According to one embodiment of the present invention, there is provided a low noise amplifier including: an input signal stage which receives an input signal; a first amplifier which has a first input terminal and a first output terminal and is configured to receive the input signal in the first input terminal, generate a first amplification signal by amplifying the received input signal, and output the generated first amplification signal, as a first output signal, to the first output terminal; a second amplifier which has a second input terminal and a second output terminal and is configured to receive the input signal in the second input terminal, generate a second amplification signal by amplifying the received input signal, and output the generated second amplification signal, as a second output signal, to the second output terminal; an output signal stage which receives the first output signal and the second output signal and outputs a superimposition signal obtained by superimposing the first output signal and the second output signal; a first resistor which feeds back the superimposition signal to the input signal stage; and a switch which connects/disconnects between the input signal stage and the output signal stage.

When the switch is switched off, the first amplifier and the second amplifier may be turned on to be operated in a high gain mode, and, when the switch is switched on, the first amplifier and the second amplifier may be turned off to be operated in a low gain mode.

The first amplifier may include a first current drawing-in terminal, a first input terminal and a first current drawing-out terminal and outputs the first output signal to the first current drawing-in terminal when the input signal is applied to the first input terminal. The second amplifier may include a second current drawing-in terminal, a second input terminal and a second current drawing-out terminal and outputs the second output signal to the second current drawing-out terminal when the input signal is applied to the second input terminal.

The first amplifier may be an NMOS (N-channel Metal Oxide Semiconductor) amplifier.

The second amplifier may be a PMOS (P-channel Metal Oxide Semiconductor) amplifier.

The low noise amplifier may further include: a third resistor which supplies a bias voltage to the first input terminal, wherein the third resistor has the resistance of 20 kΩ to 50 kΩ.

According to another embodiment of the present invention, there is provided a low noise amplifier including: an input signal stage which receives an input signal; a first capacitor which generates a first blocking signal by blocking a DC component contained in the input signal; a second capacitor which generates a second blocking signal by blocking a DC component contained in the input signal; a first transistor which has a first input terminal, a first current drawing-in terminal and a first current drawing-out terminal and is configured to receive the first blocking signal in the first input terminal, generate a first amplification signal by amplifying the received first blocking signal, and output the generated first amplification signal to the first current drawing-out terminal; a second transistor which has a second input terminal, a second current drawing-in terminal and a second current drawing-out terminal and is configured to receive the second blocking signal in the second input terminal, generate a second amplification signal by amplifying the received second blocking signal, and output the generated second amplification signal to the second current drawing-out terminal; an output signal stage which connects the first current drawing-in terminal and the second current drawing-out terminal and outputs a superimposition signal obtained by superimposing the first amplification signal and the second amplification signal; a first resistor which supplies a bias voltage to the second input terminal of the second transistor; and a fifth transistor which connects/disconnects between the input signal stage and the output signal stage.

The low noise amplifier may further include: a third transistor which has a third input terminal, a third current drawing-in terminal and a third current drawing-out terminal connected to the first current drawing-in terminal of the first transistor and is configured to output the first amplification signal to the third current drawing-in terminal; and a fourth transistor which has a fourth input terminal, a fourth current drawing-in terminal and a fourth current drawing-out terminal connected to the second current drawing-in terminal of the second transistor and is configured to output the second amplification signal to the fourth current drawing-in terminal.

The first capacitor and the second capacitor may protect the first input terminal and the second input terminal from ESD (Electrostatic Discharge), respectively.

The low noise amplifier may further include: a second resistor which supplies a bias voltage to an input terminal of the fifth transistor, wherein the second resistor has the resistance of 20 kΩ to 50 kΩ.

The low noise amplifier according to claim 8, further comprising: a third resistor which supplies a first bias voltage to the first input terminal, wherein the third resistor has the resistance of 20 kΩ to 50 kΩ.

The first resistor may have the resistance of 50Ω to 2 kΩ and provide real term impedance for impedance matching with the input signal stage.

When the fifth transistor is turned on, the low noise amplifier may operate in a low gain mode in which the input signal is transferred to the output signal stage.

When the low noise amplifier operates in the low gain mode, the third transistor and the fourth transistor may be switched such that the first current drawing-in terminal of the first transistor and the second current drawing-out terminal of the second transistor have high impedance.

The low noise amplifier may further include: a sixth transistor configured to set an initial voltage of a current drawing-in terminal of the fifth transistor to 0V when the low noise amplifier operates in the low gain mode; and a sixth resistor connected between the output signal stage and a current drawing-in terminal of the sixth transistor for impedance matching.

A fourth resistor and a fifth resistor may be connected between the third input terminal of the third transistor and the fourth input terminal of the fourth transistor; and, when fifth transistor is turned off, the low noise amplifier may operate in a high gain mode in which the input signal is amplified and output to the output signal stage, and, a bias voltage of the third input terminal of the third transistor and a bias voltage of the fourth input terminal of the fourth transistor may be varied such that the third transistor and the fourth transistor are always turned on.

According to embodiments of the present invention, it is possible to provide an inductor-less low noise amplifier (LNA) with a first amplifier and a second amplifier, which is capable of making IC (Integrated Circuit) more compact and improving a noise figure (NF) by increasing trans-conductance of the LNA by reusing a current of the first amplifier in the second amplifier.

In addition, it is possible to provide a low noise amplifier which is capable of achieving high linearity by cancelling the third-order trans-conductance of the first amplifier and the third-order trans-conductance of the second amplifier from each other in order to minimize a third-order intermodulation component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a channel environment of a conventional LNA.

FIG. 2 shows a simplified circuit of an LNA according to an embodiment.

FIG. 3 shows simulation waveforms of the third-order trans-conductance shown in FIG. 2.

FIG. 4A is a detailed circuit diagram of an LNA according to this embodiment.

FIG. 4B is a detailed circuit diagram of an LNA according to another embodiment of the present invention.

FIG. 5 is a circuit diagram of an LNA operating in a high gain mode.

FIG. 6 is a circuit diagram of an LNA operating in a low gain mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The above objects, features and advantages will become more clearly apparent from the following detailed description in conjunction with the accompanying drawings. Therefore, the technical ideas of the present invention can be easily understood and practiced by those skilled in the art. In the following detailed description of the present invention, concrete description on related functions or constructions will be omitted if it is deemed that the functions and/or constructions may unnecessarily obscure the gist of the present invention. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Throughout the drawings, the same or similar elements are denoted by the same reference numerals.

FIG. 2 shows a simplified circuit of a low noise amplifier (LNA) according to an embodiment.

Referring to FIG. 2, an LNA 200 includes an input signal stage 210, an LNA core 220, a switch S₁, a first resistor R₁ and an output signal stage 230. It is here noted that the elements included in the LNA 200 are not limited thereto.

Hereinafter, amplifiers M_(N1) and M_(P1) constituting the LNA core 220 according to this embodiment will be described on the basis of a bias current for convenience of description.

As an NMOS (N-channel Metal Oxide Semiconductor) amplifier, the first amplifier M_(N1) includes a gate, a drain and a source. On the basis of the bias current, the ‘source’ is referred to as a ‘first current drawing-in terminal,’ the ‘drain’ is referred to as a ‘first current drawing-out terminal,’ and the ‘gate’ is referred to as a ‘first input terminal.’

As a PMOS (P-channel Metal Oxide Semiconductor) amplifier, the second amplifier M_(P1) includes a gate, a drain and a source. On the basis of the bias current, the ‘source’ is referred to as a ‘second current drawing-out terminal,’ the ‘drain’ is referred to as a ‘second current drawing-in terminal,’ and the ‘gate’ is referred to as a ‘second input terminal.’

The gates of the first and second amplifiers M_(N1) and M_(P1) control a current flow between the sources and drains thereof. The drain refers to a terminal through which carriers supplied from the source pass through a channel region and are externally ejected. The source supplies carriers transporting a current. Here, the first input terminal of the first amplifier M_(N1) and the second terminal of the second amplifier M_(P1) are both connected to the input signal stage 210.

The input signal stage 210 receives a single input of an input signal RF_(in) and applies signal having the same phase to the first input terminal of the first amplifier M_(N1) and the first second terminal of the second amplifier M_(P1), respectively.

When the switch S₁ is switched off, the LNA 200 operates in a high gain mode. In the high gain mode, the first amplifier M_(N1) and the second amplifier M_(P1) amplify the input signal RF_(in). On the other hand, when the switch S₁ is switched on, the LNA 200 operates in a low gain mode. In the low gain mode, the input signal RF_(in) is output to the output signal stage 230. That is, the LNA 200 has a unity gain.

The first resistor R₁ provides a bias voltage to the second amplifier M_(P1) and feeds back an output signal RF_(out) of the output signal stage 230 to the input signal stage 210 in order to improve linearity characteristics of the LNA 200. The first resistor R₁ has the resistance of 50Ω to 2 kΩ and provides real term impedance for impedance matching between an input signal source supplying the input signal RF_(in) and the input signal stage 210 of the LNA 200.

The LNA core 220 includes the first amplifier M_(N1) and the second amplifier M_(P1). Elements included in the LNA core 220 are not necessarily limited thereto.

The first amplifier M_(N1) uses a first transistor M_(N1) as a CS (Common source) amplifier and the second amplifier M_(P1) uses a second transistor M_(P1) as a CS amplifier. The second transistor M_(P1) operates as a load resistor of the first amplifier M_(N1) and also operates as the second amplifier M_(P1) by reusing a current of the first amplifier M_(N1).

The LNA 200 may a CMOS (Complementary Metal Oxide Semiconductor) transistor. The trans-conductance g_(m) of the CMOS transistor is expressed by the following equation 1 g _(m)=√{square root over (2·μ_(n) ·C _(ox) ˜I _(d))}  [Eq. 1]

Where, μ_(n) is a mobility of charges, Cox is a capacitance of a gate oxide, and I_(d) is a drain current. According to Eq. 1, a higher trans-conductance requires a larger drain current. In one embodiment, the LNA 200 has an advantage of increasing the trans-conductance when the first amplifier M_(N1) and the second amplifier M_(P1) share a current. The trans-conductance g_(m) of the LNA 200 is expressed by the following equation 2 g _(m) =g _(m1) +g _(m2)=√{square root over (2·μ_(n) ·C _(ox) ·I _(d))}+√{square root over (2·μ_(n) ·C _(ox) ·I _(d))}  [Eq. 2]

Where, g_(m1) is the trans-conductance of the first amplifier M_(N1) and g_(m2) is the trans-conductance of the second amplifier M_(P1).

As can be seen from Eq. 2, when the first amplifier M_(N1) and the second amplifier M_(P1) share the same current, the trans-conductance of the LNA 200 can be increased without flowing an additional current. When each of the first transistor M_(N1) and the second transistor M_(P1) operates in a saturation region, the largest small signal voltage gain can be obtained. The gain of the LNA 200 is expressed by the following equation 3. A _(v)=1·(g _(m1) +g _(m2))·R ₁  [Eq. 3]

Where, g_(m1) is the trans-conductance of the first transistor M_(N1), g_(m2) is the trans-conductance of the second transistor M_(P1), and R₁ is a feedback resistor. In one embodiment, the LNA 200 can increase its gain with increase in its trans-conductance without flowing an additional current, like Eq. 2.

The noise figure (NF) of the LNA 200 according to one embodiment can be simply expressed by the following equation 4.

$\begin{matrix} {{NF} = {1 + \frac{1}{1 + A_{v}}}} & \left\lbrack {{Eq}.\mspace{14mu} 4} \right\rbrack \end{matrix}$

Where, Av is the gain of the LNA 200. According to Eq. 4, a larger gain provides a smaller NF. The LNA 200 can improve the NF characteristics by increasing a voltage gain without increasing the current I_(d), according to Eq. 2 and Eq. 3.

The LNA 200 employs a method of reducing third-order intermodulation in order to high linearity. The method of reducing third-order intermodulation is to reduce the third-order harmonic component of harmonics of an amplifier signal.

A drain current i_(ds) of CS (Common Source) MOSFET (Metal Oxide Silicon Field Effect Transistor) can be expressed as Taylor series by the following equation 5.

$\begin{matrix} {i_{ds} = {I_{dc} + {g_{m}v_{gs}} + {\frac{g_{m}^{\prime}}{2!}v_{gs}^{2}} + {\frac{g_{m}^{''}}{3!}v_{gs}^{3}}}} & \left\lbrack {{Eq}.\mspace{14mu} 5} \right\rbrack \end{matrix}$

Where, V_(GS) is a small signal gate-source voltage, g_(m) is the first-order trans-conductance, g′_(m) is the second-order trans-conductance, and g″_(m) is the third-order trans-conductance.

The LNA 200 according to one embodiment reduces the third-order trans-conductance in order to reduce the third-order intermodulation component in Eq. 5. The LNA 200 can minimize the third-order trans-conductance by cancelling the third-order trans-conductance of the first amplifier M_(N1) and the third-order trans-conductance of the second amplifier M_(P1) from each other. Accordingly, the LNA 200 can increase the linearity for an input signal by reducing the third-order harmonic component.

FIG. 3 shows simulation waveforms of the third-order trans-conductance shown in FIG. 2.

The LNA 200 increases a range of an input signal voltage by reducing the third-order harmonic component of harmonics of the input signal. As shown in FIG. 3, a two-dot chain line represents the third-order trans-conductance g″_(mN1) due to the first transistor M_(N1) as the first amplifier, a dotted line represents the third-order trans-conductance g″_(mP1) due to the second transistor M_(P1) as the second amplifier, and a solid line represents the total third-order trans-conductance. It can be seen from FIG. 3 that the third-order trans-conductance of the first amplifier and the third-order trans-conductance of the second amplifier are cancelled from each other in a range of V_(GS) from about 0.4V to about 0.55V. Accordingly, in one embodiment, the LNA 200 provides high linearity by cancelling the third-order trans-conductance of the first amplifier M_(N1) and the third-order trans-conductance of the second amplifier M_(P1) from each other in order to minimize the third-order intermodulation component.

FIG. 4A is a detailed circuit diagram of an LNA according to this embodiment.

Referring to FIG. 4A, an LNA 400 includes a capacitor part 410, an LNA core part 220, a first resistor R₁ and a fifth transistor M_(N3) acting as a first switch. When the fifth transistor M_(N3) is turned off, the LNA 400 operates in a high gain mode. In the high gain mode, the first transistor M_(N1) and the second transistor M_(P1) amplify an input signal RF_(in). On the other hand, when the fifth transistor M_(N3) is turned on, the LNA 400 operates in a low gain mode. In the low gain mode, the input signal RF_(in) is output as an output signal RF_(out). That is, the LNA 400 has a unity gain.

The capacitor part 410 includes a first capacitor C₁ which receives the input signal RF_(in) and generates a first blocking signal by blocking a DC component contained in the input signal RF_(in), and a second capacitor C₂ which receives the input signal and generates a second blocking signal by blocking a DC component contained in the input signal. The capacitor part 410 is used to improve the performance of ESD (Electro Static Discharge). More specifically, the capacitor part 410 is used to protect a first drawing-in terminal of the first transistor M_(N1) and a second drawing-in terminal of the second transistor M_(P1) from ESD.

The LNA core part 220 includes the first transistor M_(N1) and the second transistor M_(P1). The first transistor M_(N1) has a first current drawing-in terminal, a first input terminal and a first current drawing-out terminal. When the first blocking signal is applied to the first input terminal, the first transistor M_(N1) generates a first output signal, which is obtained by amplifying the first blocking signal, and outputs the first output signal, as a first amplification signal, to the first current drawing-out terminal. The second transistor M_(P1) has a second current drawing-in terminal, a second input stage and a second current drawing-out terminal. When the second blocking signal is applied to the second input terminal, the second transistor M_(P1) generates a second output signal, which is obtained by amplifying the second blocking signal, and outputs the second output signal, as a second amplification signal, to the second current drawing-out terminal.

A first bias voltage V_(n1) is applied to the first input terminal of the first transistor M_(N1) via a third resistor R₃ which is set to have high impedance (e.g., 20 kΩ to 50 kΩ) so as to have no effect on impedance matching.

The first resistor R₁ provides a bias voltage to the second amplifier M_(P1) and feeds back the output signal RF_(out) to the input signal stage in order to improve linearity characteristics of the LNA 400. The first resistor R₁ has the resistance of 50Ω to 2 kΩ and provides real term impedance for impedance matching with an input signal source supplying the input signal RF_(in).

The LNA 400 further includes a third transistor M_(N2) and a fourth transistor M_(P2). The third transistor M_(N2) has a third current drawing-out terminal connected to the current drawing-in terminal of the first transistor M_(N1), and a third current drawing-in terminal to which the first amplification signal is output. The fourth transistor M_(P2) has a fourth current drawing-in terminal connected to the current drawing-out terminal of the second transistor M_(P1), and a fourth current drawing-out terminal to which the second amplification signal is output. A superimposition RF_(out) of the first amplification signal and the second amplification signal is output to a node between the third current drawing-in terminal of the third transistor M_(N2) and the fourth current drawing-out terminal of the fourth transistor M_(P2).

A second bias voltage V_(n2) is applied to the third input terminal of the third transistor M_(N2) via a fourth resistor R₄ which is set to have high impedance (e.g., 20 kΩ to 50 kΩ).

A fifth bias voltage V_(p2) is applied to a fourth input terminal of the fourth transistor M_(P2) via a fifth resistor R₅ which is set to have high impedance (e.g., 20 kΩ to 50 kΩ).

The fifth transistor M_(N3) acts as a switch. A third bias voltage V_(n3) is applied to a fifth input terminal of the fifth transistor M_(N3) via a second resistor R₂ which is set to have high impedance (e.g., 20kΩ to 50kΩ).

The fifth transistor M_(N3) is connected between the input signal stage and the output signal stage, as shown in FIGS. 4A and 4B, or may also be connected between one end of the first capacitor C₁ and one end of the third resistor R₃ or between one end of the second capacitor C₂ and one end of the first resistor R₁.

When the fifth transistor M_(N3) is turned off, the LNA 400 operates in a high gain mode in which the first transistor M_(N1) and the second transistor M_(P1) amplify the input signal RF_(in) in a saturation region. On the other hand, when the fifth transistor M_(N3) is turned on, the LNA 400 operates in a low gain mode in which the first transistor M_(N1) and the second transistor M_(P1) are cut off. That is, the LNA 400 has a unity gain. The third bias voltage V_(n3) is applied to the fifth input terminal of the fifth transistor M_(N3) via the second resistor R₂ which is set to have high impedance (e.g., 20kΩ to 50kΩ) in order to improve linearity of the LNA 400 in the low gain mode.

When the fifth transistor M_(N3) is turned on, a sixth transistor M_(N4) is used to set an initial voltage of a fifth current drawing-in terminal of the fifth transistor M_(N3) to 0V. A fourth bias voltage V_(n4) is applied to a sixth input terminal of the sixth transistor M_(N4). A sixth resistor R₆ is set to have high impedance (e.g., 20kΩ to 50kΩ) so as to have no effect on impedance matching in the low gain mode.

FIG. 4B is a detailed circuit diagram of an LNA according to another embodiment of the present invention.

An LNA 400 of FIG. 4B has basically the same configuration as that of FIG. 4A, which will not be explained for the purpose of brevity of description, except that the first resistor R₁ is connected between the output terminal and the first input terminal of the first transistor M_(N1) and the first bias voltage Vp1 is coupled to the second input terminal of the second transistor M_(P1) via the third resistor R₃.

Hereinafter, the operation of the LNA 400 in a high gain mode and a low gain mode will be described with reference to FIGS. 5 and 6.

FIG. 5 is a circuit diagram of the LNA 400 operating in a high gain mode.

When the fifth transistor M_(N3) is turned off, all of the first transistor M_(N1), the second transistor M_(P1), the third transistor M_(N2) and the fourth transistor M_(P2) in the LNA 400 operate in a saturation region. The LNA 400 operates in the high gain mode in which the input signal RF_(in) is amplified and the output signal RF_(out) is output. The first bias voltage V_(n1), the second bias voltage V_(n2) and the fifth bias voltage V_(p2) are applied to the first input terminal of the first transistor M_(N1), the third input terminal of the third transistor M_(N2) and the fourth input terminal of the fourth transistor M_(P2), respectively, such that all of the first transistor M_(N1), the second transistor M_(P1) the third transistor M_(N2) and the fourth transistor M_(P2) operate in the saturation region in the high gain mode. As the output signal RF_(out), a self-bias voltage is applied to the second input terminal of the second transistor M_(P1).

The capacitor part 410 includes the first capacitor C₁ and the second capacitor C₂. The first capacitor C₁ receives the input signal RF_(in), generates a blocking signal by blocking a DC component contained in the input signal RF_(in), and applies the blocking signal to the first input terminal of the first transistor M_(N1). The second capacitor C₂ receives the input signal RF_(in), generates a blocking signal by blocking a DC component contained in the input signal RF_(in), and applies the blocking signal to the second input terminal of the second transistor M_(P1). The first capacitor C₁ and the second capacitor C₂ are used to improve the performance of ESD. More specifically, these capacitors C₁ and C₂ are used to protect the first drawing-in terminal of the first transistor M_(N1) and the second drawing-in terminal of the second transistor M_(P1) from ESD.

The fifth transistor M_(N3) and the sixth transistor M_(N4) operate in a cut-off region so as to have no effect on the performance of the LNA 400 in the high gain mode.

The first bias voltage V_(n1) is applied to the first input terminal of the first transistor M_(N1). The third resistor R₃ is set to have high impedance (e.g., 20kΩ to 50kΩ) so as to have no effect on impedance matching to supply the input signal RF_(in).

The first resistor R₁ provides a bias voltage to the second input terminal of the second amplifier M_(P1) and feeds back the output signal RF_(out) to the input signal stage in order to improve linearity characteristics of the LNA 400. The first resistor R₁ has the resistance of 50Ω to 2kΩ and provides real term impedance for impedance matching with the input signal source supplying the input signal RF_(in).

When the LNA 400 operates in the low gain mode, the third transistor M_(N2) and the fourth transistor M_(P2) act as switches to provide high impedance to the first transistor M_(N1) and the second transistor M_(P1). The linearity of the LNA 400 may be deteriorated due to the third transistor M_(N2) and the fourth transistor M_(P2). In order to avoid such deterioration of linearity, the fourth resistor R₄ and the fifth resistor R₅ are set to high impedance such that voltages of the third and fourth input terminals of the third and fourth transistors M_(N2) and M_(P2) follow voltages of the third and fourth current drawing-in terminals of the third and fourth transistors M_(N2) and M_(P2), respectively. A method of improving the linearity by using the high impedance for the third and fourth input terminals of the third and fourth transistors M_(N2) and M_(P2) is to use a parasitic capacitance C_(gs) such that the third transistor M_(N2) and the fourth transistor M_(P2) are always operated in a saturation region so that an input terminal voltage can follow the large input signal RF_(in), thereby preventing the third transistor M_(N2) and the fourth transistor M_(P2) from instantaneously escaping into a linear region or a cut-off region.

FIG. 6 is a circuit diagram of the LNA 400 operating in a low gain mode.

When the fifth transistor M_(N3) is turned on, all of the first transistor M_(N1), the second transistor M_(P1), the third transistor M_(N2) and the fourth transistor M_(P2) in the LNA 400 of FIG. 6 operate in a cut-off region. The LNA 400 operates in the low gain mode in which the input signal RF_(in) is output as the output signal RF_(out) with a unity gain. The third bias voltage V_(n3) and the fourth bias voltage V_(n4) are applied to the fifth input terminal of the fifth transistor M_(N3) and the sixth input terminal of the sixth transistor M_(N4), respectively, such that all of the fifth transistor M_(N3) and the sixth transistor M_(N4) operate as switches in the low gain mode. The third bias voltage V_(n3) is applied to the fifth input terminal of the fifth transistor M_(N3) via the second resistor R₂ so that the fifth transistor M_(N3) can be operated as a switch. The second resistor R₂ is set to have high impedance (e.g., 20kΩ to 50kΩ) in order to improve linearity of the LNA 400 in the low gain mode.

When the fifth transistor M_(N3) is turned on, the sixth transistor M_(N4) is used to set an initial voltage of the fifth current drawing-in terminal of the fifth transistor M_(N3) to 0V. The fourth bias voltage V₆₄ is applied to the sixth input terminal of the sixth transistor M_(N4). The sixth resistor R₆ is set to have high impedance (e.g., 20kΩ to 50kΩ) so as to have no effect on impedance matching in the low gain mode.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention. The exemplary embodiments are provided for the purpose of illustrating the invention, not in a limitative sense. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A low noise amplifier comprising: an input signal stage which receives an input signal; a first amplifier which has a first input terminal and a first output terminal and is configured to receive the input signal in the first input terminal, generate a first amplification signal by amplifying the received input signal, and output the generated first amplification signal, as a first output signal, to the first output terminal; a second amplifier which has a second input terminal and a second output terminal and is configured to receive the input signal in the second input terminal, generate a second amplification signal by amplifying the received input signal, and output the generated second amplification signal, as a second output signal, to the second output terminal; an output signal stage which receives the first output signal and the second output signal and outputs a superimposition signal obtained by superimposing the first output signal and the second output signal; a first resistor which feeds back the superimposition signal to the input signal stage; and a switch connecting between the input signal stage and the output signal stage and configured to be switched on or off, wherein when the switch is switched off, the first amplifier and the second amplifier are turned on to be operated in a high gain mode, and wherein when the switch is switched on, the first amplifier and the second amplifier are turned off to be operated in a low gain mode.
 2. The low noise amplifier according to claim 1, wherein the first amplifier includes a first current drawing-in terminal, the first input terminal and a first current drawing-out terminal and outputs the first output signal to the first current drawing-in terminal when the input signal is applied to the first input terminal.
 3. The low noise amplifier according to claim 1, wherein the second amplifier includes a second current drawing-in terminal, the second input terminal and a second current drawing-out terminal and outputs the second output signal to the second current drawing-out terminal when the input signal is applied to the second input terminal.
 4. The low noise amplifier according to claim 2, wherein the first amplifier is an NMOS (N-channel Metal Oxide Semiconductor) amplifier.
 5. The low noise amplifier according to claim 3, wherein the second amplifier is a PMOS (P-channel Metal Oxide Semiconductor) amplifier.
 6. The low noise amplifier according to claim 1, further comprising: a second resistor which supplies a bias voltage to the first input terminal, wherein the second resistor has the resistance of 20 kΩ to 50 kΩ.
 7. A low noise amplifier comprising: an input signal stage which receives an input signal; a first capacitor which generates a first blocking signal by blocking a DC component contained in the input signal; a second capacitor which generates a second blocking signal by blocking a DC component contained in the input signal; a first transistor which has a first input terminal, a first current drawing-in terminal and a first current drawing-out terminal and is configured to receive the first blocking signal in the first input terminal, generate a first amplification signal by amplifying the received first blocking signal, and output the generated first amplification signal to the first current drawing-out terminal; a second transistor which has a second input terminal, a second current drawing-in terminal and a second current drawing-out terminal and is configured to receive the second blocking signal in the second input terminal, generate a second amplification signal by amplifying the received second blocking signal, and output the generated second amplification signal to the second current drawing-out terminal; an output signal stage which connects the first current drawing-in terminal and the second current drawing-out terminal and outputs a superimposition signal obtained by superimposing the first amplification signal and the second amplification signal; a first resistor which supplies a bias voltage to the second input terminal of the second transistor; and a fifth transistor connecting between the input signal stage and the output signal stage and configured to be switched on or off.
 8. The low noise amplifier according to claim 7, further comprising: a third transistor which has a third input terminal, a third current drawing-in terminal and a third current drawing-out terminal connected to the first current drawing-in terminal of the first transistor and is configured to output the first amplification signal to the third current drawing-in terminal; and a fourth transistor which has a fourth input terminal, a fourth current drawing-in terminal and a fourth current drawing-out terminal connected to the second current drawing-in terminal of the second transistor and is configured to output the second amplification signal to the fourth current drawing-in terminal.
 9. The low noise amplifier according to claim 7, wherein the first capacitor and the second capacitor protect the first input terminal and the second input terminal from ESD (Electrostatic Discharge), respectively.
 10. The low noise amplifier according to claim 7, further comprising: a second resistor which supplies a bias voltage to an input terminal of the fifth transistor, wherein the second resistor has the resistance of 20 kΩ to 50 kΩ.
 11. The low noise amplifier according to claim 7, further comprising: a third resistor which supplies a first bias voltage to the first input terminal, wherein the third resistor has the resistance of 20 kΩ to 50 kΩ.
 12. The low noise amplifier according to claim 7, wherein the first resistor has the resistance of 50Ω to 2 kΩ and provides real term impedance for impedance matching with the input signal stage.
 13. The low noise amplifier according to claim 10, wherein, when the fifth transistor is turned on, the low noise amplifier operates in a low gain mode in which the input signal is transferred to the output signal stage.
 14. The low noise amplifier according to claim 13, wherein, when the low noise amplifier operates in the low gain mode, the third transistor and the fourth transistor are switched such that the first current drawing-in terminal of the first transistor and the second current drawing-out terminal of the second transistor have high impedance.
 15. The low noise amplifier according to claim 13, further comprising: a sixth transistor configured to set an initial voltage of a current drawing-in terminal of the fifth transistor to 0V when the low noise amplifier operates in the low gain mode; and a sixth resistor connected between the output signal stage and a current drawing-in terminal of the sixth transistor for impedance matching.
 16. The low noise amplifier according to claim 7, wherein a fourth resistor and a fifth resistor are connected between the third input terminal of the third transistor and the fourth input terminal of the fourth transistor, wherein, when fifth transistor is turned off, the low noise amplifier operates in a high gain mode in which the input signal is amplified and output to the output signal stage, and wherein a bias voltage of the third input terminal of the third transistor and a bias voltage of the fourth input terminal of the fourth transistor are varied such that the third transistor and the fourth transistor are always turned on. 